1. Field of the Invention
This invention relates to pressure-sensitive adhesive tapes, to films used in such tapes and to the use of the tapes for masking selected areas of an article during high temperature treatments, particularly for masking selected areas of electronic components such as printed circuit boards during soldering operations.
2. Description of the Related Art
Certain areas of printed circuit boards such as contact fingers and drill holes must be masked during soldering operations (for example wave soldering or hot air levelling) in such a way that no solder will penetrate into the masked area. This involves covering the selected areas with a material that will withstand the temperature of the soldering operations. In the case of wave soldering the temperature is of the order of 240.degree.-260.degree. C. for an exposure period of from 1 to 25 seconds and in the case of hot air levelling the temperatures can reach 270.degree. C. in the air stream and 250.degree. C. in the tin-lead dip. During this exposure to high temperature, the masking material must not lift from the board or shrink in any way and it must form a complete barrier against solder penetration within the masked area. The masking material must then be easily removable after soldering has taken place.
Most of the masking during wave soldering is presently achieved by use of self-adhesive tapes based on high-temperature-resistant polyimide film with a silicone-based adhesive. These tapes are removed physically from the printed circuit board after wave soldering. This is a time-consuming labour-intensive process.
It is known to wash printed circuit boards after wave soldering in order to remove flux residues. In certain cases, water has been used for this purpose. A water-soluble tape based on polyvinylalcohol is commercially available for masking selected areas of printed circuit boards. This tape is dissolved from the printed circuit board during the washing step. However, water-washing is not generally accepted as being sufficient to render a printed circuit board electronically "clean" and furthermore some residue of adhesive may remain. The use of fluorocarbon solvents, particularly those based on trichlorotrifluoroethane, is preferred for washing purposes. Such washing may suitably be carried out in an ultrasonically agitated bath of the solvent, followed by rinsing in a vapour of the solvent, or by use of a spray system in which the solvent is sprayed by jets onto the printed circuit board. After hot air levelling, it is conventional to wash the printed circuit board in methylene chloride which is a relatively power solvent. However masking materials presently in use during hot air levelling have not been very successful mainly because they are displaced by the hot air jets and they have inadequate heat resistance.
Although a solvent-soluble masking agent is commercially available in solution form, this is less satisfactory to use than a tape which can be handled and applied in predetermined shapes. Also the solution must be left for a certain period to dry and cure, which causes delay.